1. Field of the Invention
This invention relates to a method and apparatus for the drilling of the curved borehole portion of a horizontal or directional well. More specifically, but not by way of limitation, this invention relates to the drilling of short radius curved boreholes.
2. The Prior Art
It is generally known that the final portion of a rotary drilling string (the so-called drilling collar) or the like is under compressive loads and torque during drilling, while the upper portion of the drilling string is under tension. As such, the weight of the drill collar below the point of last contact with the borehole wall may be thought of as being divided into two components; one acting along the axis of the collars and the second acting normal to the first, perpendicular to the borehole.
It is also generally known that, in principle, if the downhole orientation and magnitude of the normal force could be controlled during drilling, the drill bit could essentially be steered to any desired subsurface location or strata. Although theoretically possible and highly desirable, such a process and corresponding drilling equipment to achieve such a goal have not yet been developed. However, various processes and associated equipment have been employed that generally achieve varying degrees of what is recognized in the art as directional drilling. For example, it is common practice in oil and gas well drilling to use a so-called "whipstock" (a sloped plug inserted below the drill bit) to intentionally deflect the drill bit in a desired direction, thus creating a deviation in the direction of drilling.
It is also common practice in oil and gas well drilling to employ equipment and methods to minimize or eliminate the effect of the force normal to the bore hole such as to maintain the drilling in a vertical line. Thus, various types of drilling collars, stabilizers and the like have been proposed to keep the drilling process proceeding uniformly in one direction. For example, U.S. Pat. Nos. 3,145,758 and 4,319,649 disclose drill collar stabilizers to maintain the drilling in a straight line.
In U.S. Pat. No. 4,220,213 an eccentric member having a heavy, thick walled side and a lighter, thin walled side is placed concentrically about the drill bit collar or a rigid drill string with an offset projection on the outside of the eccentric member positioned usually 90 degrees to the right of the heavy, thick walled portion. In this manner, gravity will cause the heavy, thick walled portion of the eccentric collar to rotate to the underside or low side of a deviating drill string, thus positioning the projection such as to alleviate or compensate for the undesirable "walking" of the drill bit. In other words, an eccentric tumbler member rotatably supported on the drill string is used to prevent the drill bit from moving laterally and the resulting normal force continuously restores the hole to vertical.
Various types of drilling collar stabilizers have also been proposed to alter the direction of drilling. For example, U.S. Pat. Nos. 4,305,474 and 4,465,147 disclose stabilizers that create a deflecting force perpendicular to the drill string in order to control and guide the drill bit along a desired course of direction. Also, the use of an eccentric stabilizer has been proposed in U.S. Pat. No. 4,076,084 to drill a directionally oriented hole such as commonly practiced when drilling from an offshore platform or the like.
One particularly difficult type of drilling process to control is the so-called lateral or horizontal drilling. Unlike the concept of directional drilling wherein radii in terms of hundreds of feet and deviations up to one to two miles are to be achieved relative to the surface location of the drilling platform or drilling rig, the concept of lateral drilling involves creating a highly curved well bore usually as an offshoot from a pre-drilled well bore. Thus, for example, in U.S. Pat. No. 4,402,551 a whipstock is employed to drill short radius horizontal holes below a vertical cased well bore. It is acknowledged in this patent that current, state-of-the-art techniques limit the smallest radius of curvature for surface drilling to 19 feet (i.e., 3 degrees of deflection per foot over 30 feet of linear drilling) to produce a horizontal drainhole at depths greater than 2,000 feet.
Part of the difficulty in lateral drilling was the elimination or reduction in the spiral effect that was caused by the drill bit turning to the right. This problem was first fully addressed in U.S. Pat. No. 2,712,434 whereby a non-rotating eccentric sleeve was used in conjunction with a bushing latch type mechanism which allowed the orientation back to the desired position after the drill bit had wandered off the desired track. In an attempt to maintain a constant heading, U.S. Pat. Nos. 4,739,843 and 4,699,224 addressed the problem of poor orientation control by using the aforementioned non-rotating eccentric sleeve and adding spring-loaded blades attached in order to grip the wellbore and to maintain orientation as the drilling assembly is advanced. It was advocated in these patents that periodic repositioning of the blades be accomplished so that a planar curve could be drilled.
Although the prior art has achieved some degree of success in drilling a tight radius or curvature, the failure to develop a commercial apparatus to date has been: (1) the radius of curvature is inconsistent, (2) the ball/pin flexible joint is weak and could fail under normal operating conditions, (3) maintaining the desired orientation of deflection is difficult, (4) the constant engagement of spring-loaded blades can cause multiple problems ranging from mis-orientation to sticking in the hole. In spite of previous efforts in the prior art, an acceptable method for drilling a tight radius of curvature without unacceptable deviation out of the plane of rotation has yet to be achieved. The present invention is viewed as being an improvement over the prior art methods and apparatus in that the control over the azimuth and inclination of a curved wellbore has been achieved and the inherent problems associated with the prior art have been eliminated.